ee0a49a687
__put_user_asm_goto() provides more flexibility to GCC and avoids using
a local variable to tell if the write succeeded or not.
GCC can then avoid implementing a cmp in the fast path.
See the difference for a small function like the PPC64 version of
save_general_regs() in arch/powerpc/kernel/signal_32.c:
Before the patch (unreachable nop removed):
0000000000000c10 <.save_general_regs>:
c10: 39 20 00 2c li r9,44
c14: 39 40 00 00 li r10,0
c18: 7d 29 03 a6 mtctr r9
c1c: 38 c0 00 00 li r6,0
c20: 48 00 00 14 b c34 <.save_general_regs+0x24>
c30: 42 40 00 40 bdz c70 <.save_general_regs+0x60>
c34: 28 2a 00 27 cmpldi r10,39
c38: 7c c8 33 78 mr r8,r6
c3c: 79 47 1f 24 rldicr r7,r10,3,60
c40: 39 20 00 01 li r9,1
c44: 41 82 00 0c beq c50 <.save_general_regs+0x40>
c48: 7d 23 38 2a ldx r9,r3,r7
c4c: 79 29 00 20 clrldi r9,r9,32
c50: 91 24 00 00 stw r9,0(r4)
c54: 2c 28 00 00 cmpdi r8,0
c58: 39 4a 00 01 addi r10,r10,1
c5c: 38 84 00 04 addi r4,r4,4
c60: 41 82 ff d0 beq c30 <.save_general_regs+0x20>
c64: 38 60 ff f2 li r3,-14
c68: 4e 80 00 20 blr
c70: 38 60 00 00 li r3,0
c74: 4e 80 00 20 blr
0000000000000000 <.fixup>:
cc: 39 00 ff f2 li r8,-14
d0: 48 00 00 00 b d0 <.fixup+0xd0>
d0: R_PPC64_REL24 .text+0xc54
After the patch:
0000000000001490 <.save_general_regs>:
1490: 39 20 00 2c li r9,44
1494: 39 40 00 00 li r10,0
1498: 7d 29 03 a6 mtctr r9
149c: 60 00 00 00 nop
14a0: 28 2a 00 27 cmpldi r10,39
14a4: 79 48 1f 24 rldicr r8,r10,3,60
14a8: 39 20 00 01 li r9,1
14ac: 41 82 00 0c beq 14b8 <.save_general_regs+0x28>
14b0: 7d 23 40 2a ldx r9,r3,r8
14b4: 79 29 00 20 clrldi r9,r9,32
14b8: 91 24 00 00 stw r9,0(r4)
14bc: 39 4a 00 01 addi r10,r10,1
14c0: 38 84 00 04 addi r4,r4,4
14c4: 42 00 ff dc bdnz 14a0 <.save_general_regs+0x10>
14c8: 38 60 00 00 li r3,0
14cc: 4e 80 00 20 blr
14d0: 38 60 ff f2 li r3,-14
14d4: 4e 80 00 20 blr
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/94ba5a5138f99522e1562dbcdb38d31aa790dc89.1599216721.git.christophe.leroy@csgroup.eu
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.